This project will further dissect the molecular and cellular basis of Xanthomonas campestris pathovar vesicatoria (Xcv) pathogenesis in plants. This bacterium uses a specialized secretory system to orchestrate cell-to-cell communication during the early stages of plant infection. The type III secretion system (TTSS) enables direct secretion and translocation of bacterial proteins into host plant cells. Once inside the host, TTSS effectors, acting presumably as virulence proteins, collectively modulate the cell to initiate disease symptoms in susceptible plants and to activate defense responses in resistant plants. To date, the mechanisms by which XcvTTSS effectors modulate plant physiology are virtually unknown. However, our work has provided important insights to the potential mechanisms used by the YopJ-like effectors, a family that is prevalent in Xanthomonas and conserved amongst plant and animal bacterial pathogens. Such conservation implies that pathogenic microbes use a specific mechanism to interfere with host cell signaling. We have shown that Xcv effectors in the YopJ and XopD family function as cysteine proteases inside plant and animal cells. The substrates for these effectors are highly conserved small ubiquitin-like modifiers (SUMO) that are covalently added to a number of regulatory proteins. Thus, we predict that YopJ-like effectors exert their pathogenic effect on host cells by disrupting posttranslational SUMO modification of proteins. [unreadable] [unreadable] The overall goal of this study is to identify and study the plant signal transduction pathways that are affected by the YopJ-like effector AvrBsT during Xanthomonas pathogenesis. Specifically, we will: (1) explore the effect of AvrBsT proteolysis on the SUMO pathway in Arabidopsis by isolating SUMOylated plant targets, (2) isolate the Arabidopsis bst disease resistance gene that provides protection against AvrBsT, and (3) use genetic screens to dissect the AvrBsTBST disease resistance signal transduction pathway in Arabidopsis. We believe our findings will reveal common mechanisms used by SUMO proteases conserved in both plant and animal pathogens to control eukaryotic physiology during bacterial-host interactions. [unreadable] [unreadable]